Design Details
The FBH or Force Balance Horizontal seismometer is very similar to the Inyo FBV design by Dave Nelson and Brett Nordgren except the pendulum is turned 90 degrees and will be sensitive horizontal motions. The mechanical design is complete for the first draft version. The design seems to be working so far. The seismometer is very sensitive to horizontal motion, however this also creates other problems. Even though the seismometer is located in seismic vault 3 feet down in the ground, it is susceptable to wind noise. There are trees located 15 to 30 feet away and when the wind comes up there extra noise displayed. This is the reason why a Force Balance Vertical seismometer is preferred.
The FBH has been tested with 0.002" SS, 0.001" SS, and 0.002" Kapton film for the pivot or hinge material. The 0.002" thick stainless steel did not work at all. No matter how the seismometer base was adjusted, 
the resonant period was always about 1.8 seconds. Next 0.001" stainless steel was tried. The period was 5 seconds when the pendulum beam was horizontal and the period could then be adjusted to less than 4 seconds without problem when the front base was lowered. Kapton film 0.002 was also tried. The FBH was unstable with the pendulum set to the horizontal position (this is be normal). The period could be readily adjusted by lowering the front base. One initial concern was what would happen to the pendulum when the coil was attached. Would the coil weight tend to rotate pendulum and thus require a counter weight to balance the coil. The coil is light enough such that no rotation was measured. The hinge or pivot is 0.5" wide and the gap between pendulum and vertical support is 0.012". The weight of the pendulum is 92 grams when measured at the center of the coil.
Seismometer with PCB
